In recent years, the development of automotive fuel injectors, etc., that use stacked piezoelectric elements as piezoelectric actuators has been proceeding to address automotive environmental problems and other automotive related issues such as fuel economy and exhaust emissions.
A stacked piezoelectric element generally comprises: a ceramic stack constructed by alternately stacking a piezoelectric layer made of a piezoelectric material and an internal electrode layer (or internal electrode) having electrical conductivity; and an external electrode bonded to a side face of the ceramic stack via an electrically conductive adhesive. An end face of the internal electrode layer is exposed in the side face of the ceramic body so that the end face contacts the conductive adhesive to attain electrical conduction (hereinafter sometimes simply called conduction) to the external electrode. In operation, a voltage is applied across the internal electrode layer, causing the piezoelectric layer to deform.
Such a stacked piezoelectric element is driven for use under severe conditions over a long period of time. There has therefore been the problem that the conductive adhesive may delaminate from the side face of the ceramic stack due to the stress, etc., caused by the deformation of the piezoelectric layer during operation. This has led to the possibility of a contact failure occurring between the internal electrode layer and the conductive adhesive, resulting in conduction failure. Furthermore, as the area of the end face of the internal electrode layer exposed in the side face of the ceramic stack is small, the possibility of the above failure dramatically increases in the event of delamination of the conductive adhesive.
To solve the above problem, various methods have been proposed to ensure good conduction between the internal electrode layer and the conductive adhesive.
For example, in Patent Document 1, a method is proposed that increases the area of the end face by shot blasting the side face of the stack block thereby causing the end of the internal electrode to protrude from the side face of the stack.
This method, however, involves manufacturing problems such as a difficult in quality control during volume production and the need for masking portions where shot blasting is not to be applied.
On the other hand, Patent Documents 2 and 3 each propose a method that forms an electrically conductive raised portion on the end of the internal electrode by metal plating. Further, Patent Documents 4 and 5 each propose a method that provides an electrically conductive protruding terminal on the end of the internal electrode.
However, with any of the above methods, the area of the end face of the internal electrode is nevertheless small, and there still exists the possibility that the conductive adhesive may delaminate, because the adhesive deteriorates and the adhesion drops as the piezoelectric layer is repeatedly stressed due to deformation and heat expansion during driving. Accordingly, it is difficult to solve the conduction failure problem.
The present invention has been devised in view of the above problem, and it is an object of the invention to provide a stacked piezoelectric element that has excellent durability and reliability by ensuring reliable electrical conduction between the internal electrode layer and the conductive adhesive, and a method of fabricating such a stacked piezoelectric element.